Conventionally, laser measuring devices of a wavelength modulating type have been proposed that use the self-coupling effect of semiconductor lasers (See Japanese Unexamined Patent Application Publication 2006-313080 (“JP '080”)). The structure of this type of laser measuring device is illustrated in FIG. 9. The laser measuring device of FIG. 9 includes a semiconductor laser 201 for emitting a laser beam at an object 210; a photodiode 202 for converting into an electric signal the optical power of the semiconductor laser 201: a lens 203 that focuses a beam from the semiconductor laser 201 to direct it to an object 210, and that focuses a return beam from the object 210 to inject it into the semiconductor laser 201; a first laser driver 204 for repetitively alternating between a first emission interval over which the emission wavelength of the semiconductor laser 201 increases continuously and a second emission interval over which the emission wavelength decreases continuously; an electric current/voltage converting/amplifying portion 205 for converting the outputted electric current from the photodiode 202 into a voltage, and then amplifying; a signal extracting circuit 206 for performing double differentiation on the outputted voltage of the electric current/voltage converting/amplifying portion 205; a counting device 207 for counting the number of mode hope pulses (hereinafter termed “MHPs”) included in the outputted voltage of the signal extracting circuit 206; a calculating device 208 for calculating the distance to the object 210 and the speed of the object 210; and a displaying device 209 for displaying the results of the calculations by the calculating device 208.
The laser driver 204 provides, as an injected electric current into the semiconductor laser 201, a triangle wave driving current that repetitively increases and decreases at a constant rate of change in respect to time. Doing so drives the semiconductor laser 201 so as to repetitively alternate between a first emission interval, wherein the emission wavelength increases continuously at a constant rate of change, and a second emission interval over which the emission wavelength is reduced continuously at a continuous rate of change. FIG. 10 is a diagram illustrating the change, in respect to time, of the emission wavelength of the semiconductor laser 201. In FIG. 10, P1 is the first emission interval, P2 is the second emission interval, λa is the minimum value of the emission wavelength in each of the intervals, λb is the maximum value for the emission wavelength in each of the intervals, and Tt is the period of the triangle wave.
The laser beam that is emitted from the semiconductor laser 201 is focused by the lens 203, to be incident on the object 210. The beam that is reflected from the object 210 is focused by the lens 203 to be injected into the semiconductor laser 201. The photodiode 202 converts the optical power of the semiconductor laser 201 into an electric current. The electric current/voltage converting/amplifying portion 205 converts the outputted electric current from the photodiode 202 into a voltage, and then performs amplification, and the signal extracting circuit 206 performs double differentiation on the outputted voltage from the electric current/voltage converting/amplifying portion 205. The counting device 207 counts the number of MHPs included in the outputted voltage from the signal extracting circuit 206 in the first emission interval and the second emission interval P2, separately. The calculating device 208 calculates the distance of the object 210 and the speed of the object 210 based on the minimum emission wavelength λa and the maximum emission wavelength λb the semiconductor laser 201, the number of MHPs in the first emission interval P1, and the number of MHPs in the second emission interval P2. The use of this self-coupled laser measurement device technology makes it possible to measure the number of MHPs to calculate a vibration frequency for the object from the numbers of MHPs.
The laser measuring device as described above has a problem in that there will be error in the number of MHPs that are counted by the counting device when, for example, counting noise such as external light as MHPs or when there are MHPs that are not counted due to missing signals, producing error in the physical quantities that are calculated, such as the distance and the vibrational frequency.
Given this, a counting device was proposed that is able to eliminate the effects of undercounting or overcounting at the time of counting through measuring the period of the MHPs during the counting interval, producing a distribution of the counts of the periods within the counting interval from the measurement results, calculating representative values for the periods of the MHPs from the frequency distribution, calculating, based on the frequency distribution, a total Ns of the frequencies in each bin that is no more than a first specific multiple of the representative value and calculating a total Nw of the frequencies of the bins that are no less than a second specific multiple of the representative value, and correcting the result for counting the MHPs based on these frequencies Ns and Nw (See Japanese Unexamined Patent Application Publication 2009-47676 (“JP '676”)).
The counting device disclosed in JP '676 is able to perform generally good correction insofar as the SN (signal-to-noise ratio) is not extremely low.
However, in the counting device disclosed in JP '676, in some cases a large number of signals with periods that are about one half of the actual period of the MHP, or signals with short periods, may be produced through the occurrence of chattering due to noise at frequencies higher than those of the MHPs near to a threshold value of binarization of the signals inputted into the counting device when, in a measurement of a short distance, the signal strength is extremely strong when compared to the hysteresis width. In this case, a period that is shorter than the actual period of the MHP will be used as the representative value for the distribution of periods, making it impossible to correct the MHP counting result properly, and thus there is a problem in that the MHP counting result may be, for example, several times larger than the actual value.
Given this, another counting device is proposed that is able to correct counting error even in a case wherein high-frequency noise is produced continuously in the input signal (See Japanese Unexamined Patent Application Publication 2011-33525 (“JP '525”)). The measuring device disclosed in JP '525 counts the number of run lengths of input signals during the counting interval, measures the run lengths of the input signal during the counting interval, constructs a frequency distribution of the run lengths of the input signals in the measurement period from the measurement results, calculates a representative value for the distribution of run lengths in the input signal from this frequency distribution, calculates a total Ns for the number of run lengths that are less than 0.5 times the representative value and a total Nwn number of run lengths that are no less than 2n times the representative value and less than (2n+2) times the representative value (where n is a natural number no less than 1), and corrects the MHP counting result based on these frequencies Ns and Nwn.
However, the waveform of an interference pattern such as an MHP is asymmetrical in respect to time due to the characteristics of the carrier wave-removing circuit and the state of the target object (See Japanese Patent 3282746). FIG. 11(A) is a diagram illustrating an interference waveform that is asymmetrical in this way, and FIG. 11(B) is a diagram illustrating the result of binarization of the waveform in FIG. 11(A). TH1 and TH2 in FIG. 11(A) are threshold values for binarization. When the interference waveform is asymmetrical in respect to time in this way, the duty ratio of the binarized signal will not be 0.5. Given this, the counting device disclosed in JP '525 has a problem in that the accuracy with which the count is corrected will suffer.
The counting device disclosed in JP '525 is able to correct counting error even in cases wherein high-frequency noise is produced in the input signal.
However, the counting device disclosed in JP '525 has a problem in that the accuracy of the correction to the count will suffer because the duty ratio of the signal wherein the interference waveform is binarized will not be 0.5 when the interference waveform is asymmetrical in respect to time.
The present invention was created in order to solve the problems set forth above, and the object thereof is to provide a counting device and counting method able to correct accurately the counting result even in cases wherein the signal that is inputted into the counting device is asymmetrical in respect to time.